Bimetallic strips for metal saw blades and metal band saw bands are manufactured by welding a tool steel top layer, preferably comprised of high speed tool steel, onto a support strip, e.g., by means of electron beam welding. Typically, this is followed by tempering the zone hardened in the welding by means of tempering heat treatment of the raw strip and by subsequent soft-annealing heat treatment of the raw strip in the weld seam region. The softened strip is then sent to fabrication operations such as final machining, etc.
After the sawteeth are machined and, possibly, set, the metal saw blade or metal band saw band is hardened and tempered. The hardening and tempering temperatures must be set appropriately for the tool steel or high speed tool steel alloy. Accordingly, the hardening temperature used for the bimetallic product is unusually high for the steel of the support strip and, therefore, excessively large grains can form and the material can be embrittled.
In order to withstand the high operational stresses which occur, and to do so with a long service life and with good cutting properties of the saw, the final hardness of the high speed tool steel teeth must be at least 65 HRC. The hardness of the support strip must be at least 470 HV in order to avoid undesirable edge deflection and to provide good guiding stability. For band saws, it is crucially important that the support strip have high bending fatigue strength and good toughness.
It is known that the use of carbon steels and preferably spring steels with carbon contents of 0.2-0.6% and contents of alloying elements of up to 10% is desirable for these uses. Such materials can be welded to high speed tool steel by the use of suitable methods which minimize the tendency to form:
brittle regions, or PA1 regions of inferior hardness
in the weld zone. Such brittle or soft regions can form during welding and/or during heat treatment. They result from diffusion of elements, particularly carbon, due to different concentrations or activities of the diffusing elements in the alloys of different composition.
Austrian Pat. 371,149 discloses the microalloying elements can be added to the support strip material, to avoid excessively large grain formation in the hardening process. The hardening process is directed at the high speed tool steel material and, thereby, prevents degradation of the toughness parameters.
All bimetallic saw strips for cutting metal thus have the disadvantage that, when one attempts to employ suitable hardening and tempering processes to produce good cutting properties in the top layers comprised of tool steel or high speed tool steel, it is difficult to simultaneously obtain good properties of the support strip. In particular, the bending fatigue strength of the support strip is limited. Consequently, fractures of the saw strip can occur, as a result of fatigue of the material of the support strip, even though the basic cutting properties remain satisfactory. Higher quality cutting materials can be employed such as in the cutting layers which have better cutting properties, but their use is uneconomical. This is because the service life of the support strip is much less than the service life of the sawteeth.